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Abstract

Electrophysiological experiments were performed to analyze the Na⁺/K⁺-ATPase in full-grown prophase-arrested oocytes of Xenopus laevis. If the Na⁺/K⁺-ATPase is inhibited by dihydroouabain (DHO), the resting potential of the membrane of Na⁺-loaded oocytes may depolarize by nearly 50 mV. This hyperpolarizing contribution to the resting potential depends on the degree of activation of the Na⁺/K⁺-ATPase and varies with intracellular Na⁺ activity (aⁱ_Na) and extracellular K⁺ (K⁺_o). It is concluded that variations of aⁱ_Na among different oocytes are primarily responsible for the variations of resting potentials measured in oocytes of X. laevis. Under voltage-clamp conditions, the DHO-sensitive current also exhibits dependence on aⁱ_Na that may be described by a Hill equation with a coefficient of 2. This current will be shown to be identical with the electrogenic current generated by the 3Na⁺/2K⁺ pump. The voltage dependence of the pump current was investigated at saturating values of aⁱ_Na (33 mmol/liter) and of K⁺_o (3 mmol/liter) in the range from -200 to +100 mV. The current was found to exhibit a characteristic maximum at about +20 mV. This is taken as evidence that in the physiological range at least two steps within the cycle of the pump are voltage dependent and are oppositely affected by the membrane potential.